In recent years, environmental issues such as global warming believed to be caused by an increase in CO2 have become serious. Research and development of eco-friendly solar cells utilizing sunlight as a clean energy source have been actively conducted. Among such solar cells, dye-sensitized solar cells have been gaining attention as low-cost solar cells offering improved photoelectric conversion efficiency.
A dye-sensitized solar cell is formed by laminating, for example, a transparent substrate, a transparent electrode formed on the transparent substrate, an oxide semiconductor layer carrying a dye, an electrolyte layer containing an electrolyte, and a substrate on which a counter electrode has been formed, in such order from the light incidence side. In particular, Grätzel cells are characterized by a porous oxide semiconductor layer obtained by calcinating nanofine particles of titanium oxide. The use of a porous oxide semiconductor layer results in an increase in the amount of a sensitized dye to be adsorbed, thereby improving photoabsorption performance.
In a method for producing the above dye-sensitized solar cell, for example, a porous semiconductor layer comprising titanium oxide particles is formed in advance on a transparent electrode formed on the surface of a transparent substrate and a dye is carried on the porous semiconductor layer. Next, a counter electrode is coated with a catalyst made of a platinum film or the like. The semiconductor layer and the platinum film are layered such that they face to each other. An electrolyte is injected into the space therebetween to form an electrolyte layer. The sides of the space are sealed with an epoxy resin or the like. Thus, a dye-sensitized solar cell is produced.
Liquid electrolytes have been conventionally used for the electrolyte layer. Therefore, there is a risk of liquid leakage from the electrolyte layer. This causes reduction of photoelectric conversion efficiency, which is problematic. In order to solve such problem, dye-sensitized solar cells each comprising an electrolyte layer that has been solidified to prevent liquid leakage have been suggested.
Patent Literature 1 discloses a dye-sensitized solar cell having a solid layer comprising a work electrode having a dye-coated semiconductor film, a counter electrode that is disposed so as to face the work electrode, and a high-molecular porous film sandwiched between the work electrode and the counter electrode, in which an electrolyte solution is retained in a space in the solid layer.
In addition, Patent Literature 2 discloses a dye-sensitized solar cell having a redox electrolyte and a porous semiconductor layer in which a dye is adsorbed in a space between a transparent conductive film formed on a transparent substrate and a conductive substrate, in which the redox electrolyte is supported by a three-dimensionally crosslinked high-molecular compound.
In addition, Patent Literature 3 discloses a dye-sensitized solar cell comprising an electrode layer, a porous semiconductor layer adsorbing a dye, an electrolyte layer, and an electrode layer, which are formed between paired bases, in which the electrolyte layer comprises a solid electrolyte containing a molten salt (in the form of ionic liquid). This solar cell contains ionic liquid, making it possible to reduce the viscosity of the electrolyte and improve ion conductance so as to increase photoelectric conversion efficiency. In addition, Patent Literature 3 describes high-molecular compounds that can be used for the solid electrolyte, including poly(metha)acrylates and epoxy resins.
Further, Patent Literature 4 discloses a dye-sensitized solar cell having a structure obtained by allowing a porous oxide semiconductor film formed on a substrate to adsorb a dye to form a dye-sensitized semiconductor electrode and causing an organic medium in which an electrolyte has been dissolved to come into contact with the semiconductor electrode. The organic medium is solidified with the use of a natural polymer such as carrageenin or agalose, or a derivative thereof.
In the cases of the above conventional solar cells, a solidified film layer retains liquid and thus volatilization or leakage of the electrolyte solution can be prevented to some extent. However, when such cells are kept at particularly high temperatures of 100° C. or higher, the electrolyte solution is exuded from the polymer matrix in a time-dependent manner, making it difficult to maintain a solid form at high temperatures for long time. This has been problematic. In addition, if heat lamination is employed, solar cell production is carried out at high temperature of 130° C. to 150° C. However, such high temperature treatment further causes the electrolyte solution to be exuded. Accordingly, sufficient photoelectric conversion efficiency cannot be achieved, which is problematic.